Microlithography reduction objective and projection exposure apparatus

ABSTRACT

A microlithography reduction objective formed from six mirrors arranged in a light path between an object plane and an image plane is provided. The microlithography reduction objective is characterized by having an image-side numerical aperture NA≧0.15. In some embodiments, the mirror closest to the image plane, i.e., the fifth mirror is arranged such that an image-side optical free working distance is greater than or equal to a used diameter of a physical mirror surface of the fifth mirror, a physical mirror surface being the area of a mirror where light rays from the object impinge. The fifth mirror may be arranged such that an image-side optical free working distance is greater than or equal to the sum of one-third the used diameter of the physical mirror surface on the fifth mirror and a length between 20 mm and 30 mm. The fifth mirror may be arranged such that the image-side optical free working distance is at least 50 mm, as well. The image-side free working distance is the physical distance between the vertex of the surface of the fifth mirror and the image plane. Other embodiments are described.

This is a Continuation-In-Part of U.S. application Ser. No. 09/503,640, filed Feb. 14, 2000, now U.S. Pat. No. 6,353,470.

FIELD OF THE INVENTION

The invention relates to a microlithography objective, a projection exposure apparatus containing the objective, and a method of manufacturing an integrated circuit using the same.

BACKGROUND OF THE INVENTION

Using a lithography system operating with wavelengths below 193 nm for imaging structures of below 130 nm resolution has been proposed. In fact, such lithography systems have been suggested for the extreme ultraviolet (EUV) range with wavelengths of λ=11 nm or λ=13 nm producing structures of below 100 nm. The resolution of a lithographic system is described by the following equation:

RES=k ₁ ·λ/NA

where k₁ is a specific parameter of the lithographic process, λ is the wavelength of the incident light, and NA is the image-side numerical aperture of the system. For example, if one assumes a numerical aperture of 0.2, then the imaging of 50 nm structures with 13 nm radiation requires a process with k₁=0.77. With k₁=0.64, the imaging of 35 nm structures is possible with 11 nm radiation.

For imaging systems in the EUV region, substantially reflective systems with multilayer coatings are available as optical components. Preferably multilayers of Mo/Be are used as multilayer coating systems for systems operating at λ=11 nm, whereas Mo/Si systems are used for λ=13 mm. Since the reflectivity of the multilayer coatings is approximating 70%, it is desirable to use as few optical components as possible in e.g. an EUV projection microlithography objective to achieve sufficient light intensity. Specifically, to achieve high light intensity and to allow for the correction of imaging errors, systems with six mirrors and a image side numerical aperture (NA)=0.20 have been used.

Six-mirror systems for microlithography have become known from the publications U.S. Pat. No. 5,686,728, EP 779,528 and U.S. Pat. No. 5,815,310. The projection lithography system according to U.S. Pat. No. 5,686,728 has a projection objective with six mirrors, where each of the reflective mirror surfaces has an aspherical form. The mirrors are arranged along a common optical axis in such a way that an obscuration-free light path is achieved. Since the projection objective known from U.S. Pat. No. 5,686,728 is used only for UV light with a wavelength of 100-300 nm, the mirrors of this projection objective have a very high asphericity of approximately ±50 μm as well as very large angles of incidence of approximately 38°. Even after reducing the image side aperture to NA=0.2, an asphericity of 25 μm from peak to peak remains, with a barely reduced angle of incidence. Such asphericities and angles of incidence are not practicable in the EUV region due to the high requirements for surface quality and reflectivity of the mirrors.

Another disadvantage of the objectives disclosed in U.S. Pat. No. 5,686,728, which precludes their use with wavelengths below 100 nm such as the 11 nm and 13 nm wavelengths desirable for EUV microlithography, is the short distance between the wafer and the mirror arranged next to the wafer. In the case of U.S. Pat. No. 5,686,728, due to this short distance between the wafer and the mirror next to the wafer, the mirrors could be made only very thin. Due to the extreme layer stress in the multilayer systems discussed for 11 nm or 13 nm wavelengths, such thin mirrors are very unstable.

A projection objective with six mirrors for use in EUV lithography, particularly also for wavelengths of 13 nm and 11 nm, has become known from EP 779,528. This projection objective also has the disadvantage that at least two of the six mirrors have very high asphericities of 26 and 18.5 μm. However, even in the EP 779,528 arrangement, the optical free working distance between the mirror next to the wafer and the wafer itself is so small that either instabilities occur or the mechanical free working distance is negative.

Thus, it is desirable to provide a projection objective for lithography with short wavelengths, preferably smaller than 100 nm, which does not have the disadvantages of the state of the art described above.

SUMMARY OF THE INVENTION

According to one aspect of the invention, the shortcomings of the prior art are overcome by a projection objective having an object plane and an image plane and a light path for a bundle of light rays from the object plane to the image plane. The six mirrors of the objective are arranged in the light path from the object plane to the image plane. According to the invention the mirror closest to the image plane where e.g. an object to be illuminated such as a wafer is situated is arranged in such a way that an image-side numerical aperture is NA≧0.15. In this application the image-side numerical aperture is understood to be the numerical aperture of the bundle of light rays impinging onto the image plane. Furthermore, the mirror arranged closest to the image plane of the objective is arranged in such a way that the image-side free working distance corresponds at least to the used diameter of the mirror next to the wafer. In a preferred embodiment the image-side free working distance is at least the sum of one-third of the used diameter of the mirror next to the image plane and a length between 20 and 30 mm. In an alternative embodiment the image-side free working distance is at least 50 mm. In a particularly preferred embodiment, the image-side free working distance is 60 mm. In this application the free working distance is defined as the distance of the vertex of the surface of the mirror next to the image plane and the image plane. All surfaces of the six mirrors in this application are rotational-symmetric about a principal axis (PA). The vertex of a surface of a mirror is the intersection point of the surface of a mirror with the principal axis (PA). Each mirror has a mirror surface. The mirror surface is the physical mirror surface upon which the bundle of light rays traveling through the objective from the object plane to the image plane impinge. The physical mirror surface or the used area of a mirror can be an off-axis or an on-axis mirror segment relative to the principal axis (PA).

According to another aspect of the invention, a projection objective that comprises six mirrors is characterized by an image-side numerical aperture, NA, greater than 0.15 and an arc-shaped field width, W, at the wafer in the range 1.0 mm≦W. The peak-to-valley deviation, A, of the aspheres are limited with respect to the best fitting sphere of the physical mirror surface of all mirrors by:

A≦19 μm−102 μm (0.25−NA)−0.7 μm/mm (2 mm−W).

In a preferred embodiment, the peak-to-valley distance A of the aspheres is limited with respect to the best fitting sphere of the off-axis segments of all mirrors by:

A≦12 μm−64 μm (0.25−NA)−0.3 μm/mm (2 mm−W).

According to yet another aspect of the invention, a projection objective that includes six mirrors is characterized by an image-side numerical aperture NA≧0.15 and an image-side width of the arc-shaped field W≧1 mm, and the angles of incidence AOI are limited for all rays of the light bundle impinging a physical mirror surface on all six mirrors S1, S2, S3, S4, S5, S6 by:

AOI≦23°−35°(0.25−NA)−0.2°/mm(2 mm−W)

wherein the angles of incidence AOI refer to the angle between the incident ray and the normal to the physical mirror surface at the point of incidence. The largest angle of any incident bundle of light rays occurring on any of the mirrors is always given by the angle of a bundle-limiting ray.

Preferably, an embodiment of the invention would encompass all three of the above aspects, e.g., an embodiment in which the free optical working distance would be more than 50 mm at NA=0.20 and the peak-to-valley deviation of the aspheres, as well as the angles of incidence, would lie in the regions defined above.

The asphericities herein refer to the peak-to-valley (PV) deviation, A, of the aspherical surfaces with respect to the best fitting sphere of the physical mirror surface of an specific mirror. The physical mirror surface of a specific mirror is also denoted as the used area of this specific mirror. The aspherical surfaces are approximated in the examples by using a sphere. The sphere has a center on the figure axis vertex of the mirror. The sphere intersects the asphere in the upper and lower endpoint of the used area in the meridian section. The data regarding the angles of incidence always refer to the angle between the incident ray and the normal to the physical mirror surface at the point of incidence. The largest angle of any incident bundle of light rays occurring on any of the physical mirror surfaces is always given by the angle of a bundle-limiting ray. The used diameter or the diameter of the physical mirror surface will be defined here and below as the envelope circle diameter of the physical mirror surface or the used area of a mirror, which is generally not circular.

In a preferred embodiment, the free working distance is 60 mm.

The objective can be used not only in the EUV, but also at other wavelengths, without deviating from the scope of the invention. In any respect, however, to avoid degradation of image quality, especially degradation due to central shading, the mirrors of the projection objectives should be arranged so that the light path of the bundle of light rays traveling from the object plane to the image plane is obscuration-free. Furthermore, to provide easy mounting and adjusting of the system, the physical mirror surfaces have a rotational symmetry to a principal axis (PA). Moreover, to have a compact design with an accessible aperture and to establish an obscuration-free light path of the bundle of light rays traveling from the object plane to the image plane, the projection objective device is designed in such a way that an intermediate image of the object situated in the object plane is formed after the fourth mirror. In such systems, it is possible that the aperture stop is situated in the front, low-aperture objective part, with a pupil plane conjugated to the aperture stop imaged in the focal plane of the last mirror. Such a system ensures telecentricity in the image plane.

In a preferred embodiment of the invention, the aperture stop is freely accessible and arranged in the light path from the object plane to the image plane between the second and third mirror. Good accessibility of the aperture stop is ensured when the ratio of the distance between the first and third mirror to the distance between the first and second mirror lies in the range of:

0.5<S 1 S 3/S 1 S 2<2.

As defined for the free working distance in general a distance between two mirrors is the distance of the vertices of the surfaces of these mirrors.

Furthermore, in order to prevent vignetting of the light running from the third to the fourth mirror, by the aperture stop arranged between the second and third mirror, the ratio of the distance between the second mirror and aperture stop to the distance between the third mirror and the aperture stop lies in the range:

0.5<S 2 aperture/(S 3 aperture)<2.

In such a system, the angles of incidence on the physical mirror surfaces in the front part of the objective are reduced.

An aperture stop which physically lies between the second mirror, S2, and the first mirror, S1, must be formed at least partially as a narrow ring in order to avoid clipping of light moving from S1 to S2. In such a design, there is a danger that undesirable direct light or light reflected on S1 and S2, will pass outside the aperture ring and reach the image plane and thus the wafer. However, if the aperture stop is placed in the light path between the second and third mirror and physically close to the first mirror (which can be easily achieved mechanically), an efficient masking of this undesired light is possible. The aperture stop can be designed both as an opening in the first mirror or an opening which is arranged behind the first mirror.

In another embodiment of the invention, the aperture stop is arranged on or near the second mirror. Arrangement of the aperture on a mirror has the advantage that it is easier to manufacture.

In order to ensure an obscuration-free ray path with simultaneously low angles of incidence, the ratio of the distance between the first and third mirrors (S1S3) to the distance between the first and second mirrors (S1S2) lies in the range:

0.3≦S 1 S 3/S 1 S 2≦2.0,

while the ratio of the distance between the second and third mirrors (S2S3) to the distance between the third and fourth mirrors (S3S4) lies in the range:

0.7≦S 2 S 3/S 3 S 4≦1.4.

In order to be able to make the necessary corrections of imaging errors in the six-mirror systems, in a preferred embodiment, all six mirrors are designed to be aspherical. However, an alternative embodiment whereby at most five mirrors are aspherical can simplify the manufacturing, because it is then possible to design one mirror, preferably the largest mirror, i.e., the quaternary mirror, in the form of a spherical mirror. Moreover, it is preferred that the second to sixth mirror be in a concave-convex-concave-convex-concave sequence.

In order to achieve a resolution of at least 50 nm, the design part of the rms wavefront section of the system should be at most 0.07λ and preferably 0.03 λ.

Advantageously, in the embodiments of the invention, the objectives are always telecentric on the image-side.

In projection systems, which are operated with a reflection mask, a telecentric light path on the object-side is not possible without illumination through a beam splitter, which reduces the transmission strongly. One such device is known from JP 95 28 31 16.

In systems with transmission mask, the projection objective can be telecentric on the object side. In these embodiments, the first mirror is preferably concave.

The telecentericity error in the image plane, where the wafer is situated should not exceed 10 mrad and is typically between 5 mrad and 2 mrad, with 2 mrad being preferred. This ensures that changes of the imaging ratio remain within tolerable limits over the depth of focus.

In an preferred embodiments of the invention, the six mirror objective may include a field mirror, a reducing three-mirror subsystem and a two-mirror subsystem.

In addition to the projection objective according to the invention, the invention also makes available a projection exposure apparatus that includes at least a projection objective device. In a first embodiment, the projection exposure apparatus has a reflection mask, while, in an alternative embodiment, it has a transmission mask. Preferably, the projection exposure apparatus includes an illumination device for illuminating an off-axis arc-shaped field and the system is designed as an arc-shaped field scanner. Furthermore, the secant length of the scan slit is at least 26 mm and the ring width is greater than 0.5 mm.

The invention will be described below with the aid of the drawings as examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the ring field in the object plane of the objective.

FIG. 2 illustrates an embodiment of the invention with an intermediate image, a freely accessible aperture stop between a second and third mirror, and a image side numerical aperture of 0.2.

FIG. 3 illustrates a prior art six-mirror objective arrangement for wavelengths>100 nm as disclosed in U.S. Pat. No. 5,686,728.

FIG. 4 illustrates a second embodiment of the invention with an aperture stop between the second and third mirror at the first mirror.

FIG. 5 illustrates a third embodiment of the invention with an aperture stop on the second mirror and a working distance of 59 mm.

FIG. 6 illustrates a fourth embodiment of the invention with an intermediate image, a image side numerical aperture NA of 0.28 as well as a free working distance on the image-side which is at least the sum of one-third of the useful diameter of the mirror nearest to the wafer and a length which lies between 20 and 30 mm.

FIG. 7 illustrates a fifth embodiment of the invention of a system with an intermediate image and a image side numerical aperture NA of 0.30.

FIGS. 8A and 8B show the used diameter for different physical mirror surfaces or used areas of a mirror

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 the object field 1100 of a projection exposure apparatus in the object plane of the projection objective according to the invention is shown. The object plane is imaged by means of the projection objective in an image plane, in which a light sensitive object, for example a wafer with a light sensitive material is arranged. The image field in the image plane has the same shape as the object field. The object (or the image) field 1100 has the configuration of a segment of a ring field, and the ring field has an axis of symmetry 1200.

In addition the axis 1200 extending to the object plane, the x-axis and the y-axis are depicted. As can be seen from FIG. 1, the axis of symmetry 1200 of the ring field runs in the direction of the y-axis. At the same time the y-axis coincides with the scanning direction of an projection exposure apparatus, which is designed as a ring field scanner. The x-direction is thus the direction that stands perpendicular to the scanning direction, within the object plane. The ring field has a so called ring field radius R, which is defined by the distance of the central field point 1500 of the image field from the principal axis (PA) of the projection objective. The arc-shaped field in the object plane as well as in the image plane has a arc shaped field width W, which is the extension of the field in scanning or y-direction and a secant length SL.

In FIGS. 2, 4 and 5, arrangements of the six-mirror projection objectives according to the invention are shown. Each embodiment has a free working distance that corresponds at least to the used diameter of the physical mirror surface or mirror segment next to the wafer. In contrast, FIG. 3 shows a prior art system for use with wavelengths>100 nm, such as the system of U.S. Pat. No. 5,686,728. In all embodiments below, the same reference numbers will be used for the same components and the following nomenclature will be employed:

first mirror (S1), second mirror (S2), third mirror (S3), fourth mirror (S4),

fifth mirror (S5), and sixth mirror (S6).

In particular, FIG. 2 shows a six-mirror projection objective with a ray path from the object plane 2, i.e. reticle plane to the image plane 4, i.e. wafer plane. The embodiment includes a field mirror S1, which forms a virtual image of an object with an imaging ratio β>0. A three-mirror system formed from S2, S3 and S4 is also provided and produces a real, reduced image of the virtual image as the intermediate image, Z. Lastly, a two-mirror system S5, S6, images the intermediate image Z in the wafer plane 4 while maintaining the requirements of telecentricity. The aberrations of the three-mirror and two-mirror subsystems are balanced against one another so that the total system has a high optical quality sufficient for integrated circuit fabrication applications.

The physical aperture stop B is arranged between the second mirror S2 and the third mirror S3. And, as is clear from FIG. 2, the aperture stop is accessible in the ray path between the second mirror S2 and the third mirror S3. Furthermore, the distance between the vertex V5 of the surface of the mirror next to the wafer, i.e., the surface of the fifth mirror S5 in the present embodiment, and the image plane is greater than the used diameter of the physical mirror surface of mirror S5. The used diameter of a physical mirror surface is explained in more detail in the description of FIGS. 8A and 8B. In other words, the following condition is fulfilled:

the physical distance from the vertex V5 of the surface of mirror S5 to the image plane 4 is greater than the used diameter of mirror S5.

Other distance requirements are also possible and may be used, such as the physical distance is (1) greater than the sum of one-third of the used diameter of the mirror next to the wafer, S5, and 20 mm, or (2) greater than 50 mm. In the preferred embodiment, the physical distance is 60 mm.

Such a physical distance guarantees a sufficiently free working distance A, and allows the use of optical components compatible for use with wavelengths<100 nm, and preferably wavelengths of 11 to 13 nm. Optical components in this range include, for example, Mo/Si or Mo/Be multilayer systems, where the typical multilayer systems for λ=13 nm is Mo/Si layer pairs and for λ=11 nm, is Mo/Be systems, both of approximately 70 layer pairs. Reflectivities attainable in such systems are approximately 70%. In the multilayer layer systems, layer stresses of above 350 MPa may occur. Stresses of such values may induce surface deformation, especially in the edge regions of the mirror.

The systems according to the invention, as they are shown, for example, in FIG. 1, have:

RES=k ₁ λ/NA.

This results in a nominal resolution of at least 50 nm and 35 nm at a minimum numerical aperture of NA=0.2 for k₁=0.77 and λ=13 nm, and for k₁=0.64 and λ=11 nm, respectively, where k₁ is a parameter specific for the lithographic process.

Furthermore, the light path for a bundle of light rays running from the object plane to the image plane of the objective shown in FIG. 2 is obscuration-free. For example, in order to provide image formats of 26×34 mm² or 26×52 mm², the projection objectives according to the invention are preferably used in an arc-shaped field scan projection exposure apparatus, wherein the secant length of the scan slit is at least 26 mm.

Numerous masks can be used in the projection exposure apparatus. The masks or reticle are arranged in the object plane of the projection objective. The masks include transmission masks, stencil masks and reflection masks. The projection objective, which is telecentric on the image side, i.e. in the image plane, can be telecentric or non-telecentric on the object side, i.e. in the object plane depending on which mask is used. For example, if the bundle of light rays is telecentric on the object-side when using a reflection mask, a transmission-reducing beam splitter must be employed. If the bundle of light rays is non-telecentric on the object-side, unevennesses of the mask leads to dimensional errors in the image. Therefore, the angle of incidence of the chief ray of the bundle of light rays through the central field point 1500 in the object plane is preferably below 10°, so that the requirements for reticle evenness lies in an achievable range. Moreover, the system of FIG. 2 which is telecentric on the image side has an image-side error of telecentry at the wafer level of 1 mrad for a image side numerical aperture of 0.2.

Due to the high image-side telecentricity, the entrance pupil of the last mirror S6 is at or near the focal plane of this mirror. Therefore, in systems with an intermediate image as described before, the aperture, B, is in the front, low-aperture objective part preferably in the light path between the first and third mirror S1, S3. Thus the pupil plane conjugated with the aperture stop will be imaged in the focal plane of the last mirror.

All mirrors S1-S6 of FIG. 2 are designed to be aspherical, with a maximum asphericity of approximately 7.3 μm. The low asphericity of the embodiment shown in FIG. 2 is advantageous from a manufacturing point of view, since the technological difficulties in processing the surfaces of the multilayer mirrors increases proportionally with aspherical deviation and gradient of the asphere.

The highest angle of incidence of a ray impinging a mirror surface in the six-mirror objective shown in FIG. 2 occur on the fifth mirror S5 and is approximately 18.4°. The maximum variation of the angles of incidence of the rays within a bundle of light rays impinging onto a mirror surface occurs on mirror surface of mirror S5 and is approximately 14.7°. The wavefront error at λ=13 nm is better than 0.032λ; the centroid distortion of the point spread function is <3 mm; and the static, dimension-corrected distortion lies at 4 nm.

A freely accessible aperture stop between the second and third mirror as well as no vignetting of the bundle of light rays running from S3 to S4 by the aperture stop is achieved with small angles of incidence of the rays impinging onto the mirror surfaces when the following distance conditions are fulfilled:

 0.5<S 1 S 3/S 1 S 2<2

and

0.5<S 2 aperture/(S 3 aperture)<2.

Here, the abbreviation S1S3 means the mechanical distance or physical distance between the vertices V1 and V3 of the surface of the mirrors S1 and S3. And, “S2 aperture” means the mechanical distance between the vertex V2 of the surface of mirror S2 and the aperture. Furthermore, in order to reduce the angles of incidence on the mirrors in any of the embodiments of FIGS. 2, 4, and 5, the distance from the object plane, where for example the reticle is situated to the vertex of the surface of the mirror S1, is made smaller than the mechanical distance from the vertex of the surface of mirror S2 to the vertex of the surface of mirror S3, i.e., the following applies:

reticle S 1<S 2 S 3.

To ensure a sufficient free working distance A not only on the image side but also on the object side the reticle is situated sufficiently far in front of the first mirror next to the object plane, which is in the present case the surface of the second mirror S2. In the present case, for example, the physical distance between the reticle and the vertex V2 of the surface of mirror S2 is 80 mm.

Furthermore, in the embodiments of FIGS. 2 and 4 to 6, the physical distance between the mirrors S3 and S6 is chosen so that mirrors of sufficient thickness can be used. Thicker mirrors have sufficient strength and stability properties that can withstand the high layer tensions described above. In these systems, the following relationship is preferred:

0.3 (used diameter S 3+used diameter S 6)<S 3 S 6.

Here S3S6 denotes the physical distance between the vertex V3 of the surface of mirror S3 and the vertex V6 of the surface of the mirror S6.

In the following table, the parameters of the system represented in FIG. 2 are exemplarily shown in Code V(™) nomenclature. The objective is a 5× system with a 26×2 mm² arc-shaped field in the image plane, wherein 26 mm is the secant length of the arc-shaped field and 2 mm is the width W of the arc shaped field. Furthermore the numerical aperture is 0.2 on the image side. The mean image side radius of the system is approximately 26 mm.

TABLE 1 element No. radius Thickness diameter Type object INF 80.9127 258.1723 413.0257 S1 A(1) −88.8251 197.5712 REFL 195.6194 −324.2006 188.6170 0.0000 S2 A(2) 324.2006 188.7078 REFL aperture 67.1796 423.6214 183.2180 0.0000 S3 A(3) −423.6214 184.7062 REFL 519.0546 −74.9270 S4 A(4) 498.5484 541.0453 REFL 248.6244 109.8242 177.5488 281.5288 S5 A(5) −281.5288 65.0842 REFL S6 A(6) 281.5288 187.9549 REFL 78.3999 image image width 59.9202 53.9889 aspherical constants: Z = (CURV) Y²/[1 + (1 − (1 + K) (CURV)² Y²)^(½)] + (A)Y⁴ + (B)Y⁶ + (C)Y⁸ + (D)Y¹⁰ asphere CURV K A B C D A(1) 0.00031800 −27.686599  0.00000E+00 1.32694E−15  2.00546E−20 −8.49471E−25  A(2) 0.00094928 −3.998204  0.00000E +00 4.03849E−15 −6.15047E−20 2.73303E−25 A(3) 0.00126752 0.424198 0.00000E+00 1.58766E−15 −8.27965E−20 2.80328E−24 A(4) 0.00123850 0.023155 0.00000E+00 2.46048E−17 −1.08266E−22 3.75259E−28 A(5) 0.00329892 2.902916 0.00000E+00 1.55628E−12 −6.71619E−17 −5.30379E−21  A(6) 0.00277563 0.072942 0.00000E+00 2.96285E−16  3.99125E−21 4.55007E−26 Reference wavelength = 13 nm

FIG. 3 shows an arrangement of a projection objective for microlithography with a wavelength of λ<100 nm according to U.S. Pat. No. 5,686,728. By way of a simplified explanation and comparison only, components similar to those of FIG. 1 are provided with the same reference numbers. As is clear, the physical distance between the vertex V5 of the surface of the mirror next to the image plane S5 and the image plane, where the wafer is situated, is significantly smaller than the used diameter of the fifth mirror S5, lying mainly in the range of approximately 20 mm. This leads to strength and stability problems for the optics in the EUV region because of the extreme tensions in the layers. Furthermore, the system has very high asphericities of ±50 μm and a maximum angle of incidence of 38°. From a manufacturing and coating technology point of view, such asphericities and angles of incidence are incompatible for use in the EUV region.

FIG. 4 is an alternative embodiment of a six-mirror system in which the aperture stop is situated on the first mirror. The same components as in FIG. 2 again receive the same reference number in FIG. 4. The free working distance A to the wafer is 60 mm in this embodiment, as it was in the embodiment of FIG. 2, and thus it is greater than the used diameter of the mirror next to the wafer, S5. Similarly, as with FIG. 2, the physical distance between the vertex V2 of the surface of mirror S2 and the vertex V3 of the surface of mirror S3 was increased significantly in comparison to that of U.S. Pat. No. 5,686,728, so that large angles of incidence can be avoided in the system.

One difference to the objective of FIG. 2, is that in FIG. 4 the aperture stop B is placed on the first mirror S1. As a result of this position, a reduction in vignetting from the light reflected on S2 is possible, whereas with the physical aperture stop positioned between S1 and S2 light of the bundle of light rays running thorough the objective could pass above the aperture stop which is designed as a narrow ring. In the embodiment shown in FIG. 4, the aperture can be either an opening in the S1 mirror or an aperture disposed behind SI close to this mirror.

Another advantage of this embodiment is the spherical design of mirror S4, which presents advantages especially from the point of view of manufacturing, because mirror S4 is the largest mirror of the system. With such a design, the asphericity in the used range is increased slightly to 10.5 μm. The largest angle of incidence occurs on mirror S5 and is approximately 18.6°. The wavefront error of the arrangement is 0.032 λ, within a 1.7 mm wide arc-shaped field at λ=13 nm. Furthermore, if the mirror S4 is designed to be slightly aspherical with 0.4 μm, then the wavefront error can be kept to 0.031λ within a 1.8 mm wide arc-shaped field at λ=13 nm. Efficient masking of the undesirable light is obtained not only when the aperture stop is formed directly on mirror S1, but also when it is arranged behind, i.e., after, mirror S1. Preferably, the aperture stop is positioned such that the following relationship is obtained:

S 2 S 1≦0.9×S 2 aperture.

S2S1 denotes the mechanical distance of the vertex V2 of the surface of mirror S2 and the vertex V1 of the surface of the mirror S1.

Table 2 shows the constructional data of the 5× objective according to FIG. 4 in Code V(™) nomenclature, where the fourth mirror S4 is spherical. The mean radius of the 26×1.7 mm image field is approximately 26 mm.

TABLE 2 element No. radius Thickness diameter type Object INF 85.2401 256.1389 358.4668 S1 A(1) 0.0024 203.8941 REFL 203.8845 −358.4691 201.9677 0.0000 S2 A(2) 358.4691 201.9942 REFL aperture 60.7572 390.5456 187.2498 0.0000 S3 A(3) −390.5456 188.9474 REFL 505.8686 −104.1273 S4 A(4) 494.6729 550.3686 REFL 256.9217 114.3062 181.7337 281.6969 S5 A(5) −281.6969 64.4286 REFL S6 A(6) 281.6969 187.8549 REFL 78.1545 image image width 60.0041 53.6996 aspherical constants: Z = (CURV)Y²/[1 + (1 − (1 + K) (CURV)²Y²)^(½)] + (A)Y⁴ + (B)Y⁶ + (C)Y⁸ + (D)Y¹⁰ asphere CURV K A B C D A(1) 0.00035280 −58.238840 0.00000E + 00 2.14093E − 15 2.29498E − 20 0.00000E + 00 A(2) 0.00097971 −4.160335 0.00000E + 00 1.54696E − 15 8.15622E − 21 0.00000E + 00 A(3) 0.00117863 −2.136423 0.00000E + 00 −1.78563E − 16 3.45455E − 20 0.00000E + 00 A(4) 0.00124362 0.000000 0.00000E + 00 0.00000E + 00 0.00000E + 00 0 00000E + 00 A(5) 0.00338832 2.909987 0.00000E + 00 7.90123E − 13 7.04899E − 17 0.00000E + 00 A(6) 0.00278660 0.062534 0.00000E + 00 2.79526E − 16 7.00741E − 21 0.00000E + 00 Reference wavelength = 13 nm

Another embodiment is shown in FIG. 5, where again the same reference numbers are used for the same components as in the previous figures. Here, the aperture stop B is placed optically and physically on the secondary mirror or second mirror S2. The ability to place the aperture stop on S2 makes manufacturing easier. Therefore this arrangement is advantageous. The system of FIG. 5 is a 4× reduction system with a wavefront error of 0.021λ within a 2 mm wide image side arc-shaped field at λ=13 nm. The maximum asphericity in the used range lies at 11.2 μm, and the largest angle of incidence, which occurs at S5, is approximately 18.3°. The ring field radius R as defined in FIG. 1 of the arc-shaped field in the image plane is approximately 26 mm, as with the previous two embodiments. Furthermore, the distance between the image plane and the vertex V5 of the surface of the mirror next to the image plane, S5, is greater than the used diameter of the mirror next to the wafer, S5, and lies at around 59 mm in this embodiment.

Table 3 shows the optical parameters of the embodiment of FIG. 5 in Code V(™) nomenclature.

TABLE 3 element No. Radius thickness diameter Type object INF 84.0595 205.6642 473.5521 S1 A(1) −145.8261 147.3830 REFL 136.4700 −327.7260 aperture 112.0176 0.0000 S2 A(2) 473.5521 112.1228 REFL 163.5236 190.4830 184.4783 0.0000 S3 A(3) −190.4830 185.3828 REFL 358.6720 −399.1713 S4 A(4) 589.6560 654.5228 REFL 310.1977 207.5220 175.3066 276.2668 S5 A(5) −276.2668 65.2138 REFL S6 A(6) 276.2668 182.8159 REFL 77.5085 image image width 59.0000 53.9968 aspherical constants: Z = (CURV)Y²/[1 + (1 − (1 + K) (CURV)²Y²)^(½)] + (A)Y⁴ + (B)Y⁶ + (C)Y⁸ + (D)Y¹⁰ asphere CURV K A B C D A(1) 0.00015851 441.008070 0.00000E + 00 −3.49916E − 16 1.27478E − 19 −3.37021E − 25 A(2) 0.00089932 −5.032907 0.00000E + 00 −6.95852E − 15 −7.53236E − 20 −2.74751E − 24 A(3) 0.00188578 0.913039 0.00000E + 00 −1.60100E − 15 −9.53850E − 20 1.30729E − 26 A(4) 0.00108147 0.038602 0.00000E + 00 2.48925E − 18 −5.29046E − 24 −4.37117E−31 A(5) 0.00269068 7.253316 0.00000E + 00 −5.70008E − 13 −9.32236E − 17 −6.09046E − 21 A(6) 0.00281036 0.150957 0.00000E + 00 1.30822E− 15 1.86627E − 20 5.08158E − 25 Reference wavelength = 13 nm

FIG. 6 shows an embodiment of the invention which includes a field mirror S1, a first subsystem with the second to fourth mirror S2-S4 and a second subsystem with the fifth and sixth mirror, S5, S6. The field mirror S1 with imaging ratio,β, β>0 produces a virtual image of the object in the object plane 2. The virtual image is then imaged by the first subsystem consisting of the second, third and fourth mirrors, S2, S3, S4, which has β<0, producing a real intermediate image Z in a plane conjugate to the object plane 2. The real intermediate image Z is imaged as a real image into image plane 4 by the second subsystem, which consists of the fifth and sixth mirrors, S5, S6. The image side numerical aperture of the system is NA=0.28. The optical free working distance,s A, between the vertex of the surface of the last mirror S5 and the image plane 4 corresponds to at least the sum of one-third of the used diameter of the mirror nearest to the image plane and a length which lies between 20 and 30 mm. The aperture stop B is situated on the second mirror S2.

Table 4 shows the optical parameters of the embodiment of FIG. 6 in Code V(™) nomenclature.

TABLE 4 element No. Radius thickness Diameter Type Object INF 151.2625 194.7605 229.0820 S1 A(1) −39.4068 162.9862 REFL 147.1426 −189 6752 aperture 65.0637 0.0000 S2 A(2) 229.0820 65.1650 REFL 168.3504 137.5708 230.5128 0.0000 S3 A(3) −137.5708 234.0072 REFL 386.2567 −300.3445 S4 A(4) 437.9153 630.7784 REFL 343.1578 133.0981 257.0225 353.0840 S5 A(5) −353.0840 79.9521 REFL S6 A(6) 353.0840 264.2853 REFL 78.6376 image image width 44.0000 54.0051 aspherical constants: Z = (CURV)Y²/[1 + (1 + (1 + K) (CURV)²Y²)^(½)] + (A)Y⁴ + (B)Y⁶ + (C)Y⁸ + (D)Y¹⁰ + (E)Y¹² + (F)Y¹⁴ + (G)Y¹⁶ + (H)Y¹⁸ + (J)Y²⁰ K A B C D asphere CURV E F G H J A(1) −0.00080028 0.000000 −3.35378E − 09 5.36841E − 14 −7.86902E − 19 −5.07886E − 24 0.00000E + 00 0.00000E + 60 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(2) 0.00040002 0.000000 1.68187E − 08 2.05570E − 12 2.42710E − 16 5.69764E − 20 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(3) 0.00113964 −2.760663 0.00000E + 00 −3.55779E − 15 1.03888E − 19 −3.64996E − 24 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(4) 0.00128753 0.019273 0.00000E + 00 5.82746E − 18 −1.77496E − 22 1.64954E − 27 −6.20361E − 33 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(5) 0.00373007 11.6888968 0.00000E + 00 −5.53902E − 12 −4.32712E − 16 −1.54425E − 19 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(6) 0.00240387 −0.002567 0.00000E + 00 −6.78955E − 16 −8.39621E − 21 −2.95854E − 25 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 Reference wavelength = 13 nm

FIG. 7 shows a similar, yet alternative, embodiment to that of FIG. 6 with a six-mirror objective with field mirror S1 as well as first and second subsystems as shown in FIG. 6. The embodiment shown in FIG. 7 comprises as the embodiment in FIG. 6 an intermediate image Z. Furthermore the aperture B is formed on the second mirror S2 similar and the numerical aperture on the image side is NA=0.30. The optical parameters of this alternative embodiment are shown in Table 5 in Code V(™) nomenclature.

TABLE 5 element No. radius thickness Diameter type object INF 103.2808 197.1874 219.3042 S1 A(1) −39.2890 157.6222 REFL 142.1492 −180.0152 aperture 67.2659 0.0000 S2 A(2) 219.3042 67.4347 REFL 167.6895 131.2051 228.0182 0.0000 S3 A(3) −131.2051 232.3162 REFL 401.4441 −247.5850 S4 A(4) 378.7901 613.5493 REFL 355.7774 134.4001 268.3735 348.5086 S5 A(5) −348.5086 81.5255 REFL S6 A(6) 348.5086 269.2435 REFL 75.4983 image image width 36.1195 53.9942 aspherical constants: Z = (CURV) Y²/[1 + (1 − (1 + K) (CURV)²Y²)^(½)] + (A)Y⁴ + (B)Y⁶ + (C)Y⁸ + (D)Y¹⁰ + (E)Y¹² + (F)Y¹⁴ + (G)Y¹⁶ + (H)Y¹⁸ + (J)Y²⁰ K A B C D asphere CURV E F G H J A(1) −0.00061615 0.000000 −5.19402E − 09 1.09614E − 13 −3.44621E − 18 1.58573E − 22 −7.07209E − 27 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(2) 0.00066911 0.000000 1.69112E − 08 2.39908E − 12 2.89763E − 16 1.00572E − 19 1.84514E − 29 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(3) 0.00140031 0.000000 −8.71271E − 10 −1.47622E − 15 −3.40869E − 20 4.32196E − 24 −2.23484E − 28 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(4) 0.00143731 0.000000 2.18165E + 12 2.65405E − 17 −2.01757E − 22 1.14856E − 28 1.49857E − 32 −8.61043E − 38 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(5) 0.00378996 0.000000 8.54406E − 08 2.25929E − 12 3.36372E − 16 1.92565E − 20 5.75469E − 24 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 A(6) 0.00246680 0.000000 −3.61754E − 12 −8.29704E − 16 −1.53440E − 20 −2.24433E − 25 5.91279E − 30 0.00000E + 00 0.00000E + 00 0.00000E + 00 0.00000E + 00 Reference wavelength = 13 nm

FIGS. 8A and 8B define the used diameter D as used in the description of the above embodiments. As a first example, the illuminated field 100 on a mirror in FIG. 8A is a rectangular field. The illuminated field corresponds to the area on a mirror onto which a bundle of light rays running through the objective from the object side to the image side impinge. The used diameter D according to FIG. 8A is then the diameter of the envelope circle 102, which encompasses the rectangle 100, where the corners 104 of the rectangle 100 lie on the envelope circle 102. A more realistic example is shown in FIG. 8B. The illuminated field 100 has a kidney shape, which is expected for the physical mirror surfaces of the mirrors S1-S6 or the so called used areas of the mirrors S1-S6, when the field in the image plane as well as the field in the object plane is an arc shaped field as depicted in FIG. 1. The envelope circle 102 encompasses the kidney shape fully and it coincides with the edge 110 of the kidney shape at two points, 106, 108. The used diameter D of the physical mirror surface or the used area of the mirrors S1-S6 is then given by the diameter of the envelope circle 102.

Thus, the invention provides a six-mirror projection objective with an imaging scale of preferably 4×, 5× or 6× for use in an EUV projection system. Other uses may be employed, however. The six-mirror projection objective has the resolution required for the image field, which is e.g. arc-shaped and has a advantageous structural design, since the aspheres of the mirror surfaces are relatively low, the angles of incidence of the rays of the bundle of light rays impinging the mirror surfaces are small, and there is enough room for mounting the mirrors. 

I claim:
 1. A projection objective with an object plane and an image plane and a light path for a bundle of light rays from the object plane to the image plane for use in short wavelength microlithography, comprising; six mirrors (a first mirror (S1), a second mirror (S2), a third mirror (S3), a fourth mirror (S4), a fifth mirror (S5) disposed closest to the image plane and a sixth mirror (S6)) arranged in the light path such that an image-side numerical aperture (NA) is NA≧0.15, wherein each mirror has a physical mirror surface and wherein said fifth mirror is positioned in the light path such that at least one of the following conditions is satisfied: an image-side optical free working distance is greater than or equal to a used diameter D of a physical mirror surface of the fifth mirror; the image-side optical free working distance is greater than or equal to a sum of one-third of the used diameter of the physical mirror surface of the fifth mirror and a length between 20 mm and 30 mm; and the image-side optical free working distance is at least 50 mm; wherein the image side free working distance is the physical distance between the vertex of the surface of the fifth mirror and the image plane and wherein the physical mirror surface of a mirror is the area of the surface of a mirror, where the rays of the bundle of light rays running from the object side to the image side impinge.
 2. A projection objective with an object plane and an image plane and a light path for a bundle of light rays from the object plane to the image plane for use in short wavelength microlithography, comprising; six mirrors (a first mirror (S1), a second mirror (S2), a third mirror (S3), a fourth mirror (S4), a fifth mirror (S5) disposed closest to the image plane, and a sixth mirror (S6)) arranged in the light path such that an image-side numerical aperture (NA) is NA≧0.15 and an image side arc-shaped field width (W) lies in the range of 1.0 mm≦W, and wherein each of said six mirrors has a physical mirror surface with a maximum aspherical peak-to-valley (PV) deviation (A) from a best-fitting sphere in the following range: A≦19 μm−102 μm (0.25−NA)−0.7 μm/mm (2 mm−W) wherein the physical mirror surface of a mirror is the area of the surface of a mirror where the rays of the bundle of light rays running from the object side to the image side impinge.
 3. A projection objective with an object plane and an image plane and a light path for a bundle of light rays from the object plane to the image plane for use in short wavelength microlithography, comprising; six mirrors (a first mirror (S1), a second mirror (S2), a third mirror (S3), a fourth mirror (S4), a fifth mirror (S5) disposed closest to the image plane, and a sixth mirror (S6)) arranged such that an image-side numerical aperture (NA) is NA≧0.15 and an image-side arc-shaped field width (W) lies in the range of 1.0 mm≦W, and wherein the rays of a bundle of light rays incident on each physical mirror surface of the six mirrors have angles of incidence (AOI) relative to the surface normal of said physical mirror surface that are lying in the range: AOI≦23°−35°(0.25−NA)−0.2°/mm(2 mm−W)  wherein the physical mirror surface of a mirror is the area of the surface of a mirror where the rays of the bundle of light rays running from the object side to the image side impinge.
 4. A projection objective with an object plane and an image plane and a light path from the object plane to the image plane for use in short wavelength microlithography, comprising; a field mirror (S1) with an imaging ratio β₁>0, wherein the field mirror produces a virtual image of an object in the object plane; a first subsystem with a second mirror (S2), a third mirror (S3), and a fourth mirror (S4), wherein the first subsystem has an imaging ratio β₂<0, and wherein said first subsystem images the virtual image to a real intermediate image; a second subsystem with a fifth mirror (S5) disposed closest to the image plane and a sixth mirror (S6), wherein the second subsystem images the intermediate image to a real image in the image plane; and wherein the objective has an image-side numerical aperture (NA) and an arc-shaped field width (W) in the image plane, where an object to be illuminated is situated, and wherein each mirror has a physical mirror surface that is the area of the surface of the mirror where the rays of the bundle of light rays running from the object side to the image side impinge.
 5. A projection objective according to any one of claims 1 to 4, wherein the surface of the mirrors have a rotational symmetry with respect to a principal axis (PA).
 6. A projection objective according to one of claims 1 to 4, further comprising an aperture stop (B) in the light path positioned between the second mirror (S2) and the third mirror (S3).
 7. A projection objective according to claim 6, wherein a ratio of a physical distance between the vertex of the surface of the first mirror and the vertex of the surface of the third mirror (S1S3) to a physical distance between the vertex of the surface of the first mirror and the vertex of the surface of the second mirror (S1S2) is in the range of: 0.5<S 1 S 3/S 1 S 2<2.
 8. A projection objective according to claim 7, wherein a ratio of a physical distance between the vertex of the surface of the second mirror and the vertex of the surface of the third mirror (S2S3) to a physical distance between the vertex of the surface of the third mirror and the vertex of the surface of the fourth mirror (S3S4) lies in the range:  0.7<S 2 S 3/S 3 S 4<1.4.
 9. A projection objective according to any one of claims 1 to 4, wherein all physical mirror surfaces are aspherical.
 10. A projection objective according to any one of claims 1 to 4, wherein at most five physical mirror surfaces are aspherical.
 11. A projection objective according to any one of claims 1 to 4, wherein the second mirror, third mirror, fourth mirror, fifth mirror, and sixth mirror are in a concave-convex-concave-convex-concave sequence.
 12. A projection objective according to any one of claims 1 to 4, wherein a physical distance between the vertex of the surface of the third mirror and the vertex of the surface of the sixth mirror (S3S6) satisfies the following relationship: 0.3 (used diameter of third mirror S 3+used diameter of sixth mirror S 6)<S 3 S
 6. 13. A microlithography projection exposure apparatus comprising: a projection objective according to one of the claims 1 to 4; and an illumination system comprising a radiation source providing a bundle of light rays illuminating an arc-shaped field in the object plane of the projection objective, wherein the projection objective images a mask located in the object plane into the image plane of the projection objective, where a light sensitive objective is situated.
 14. A method for producing a microelectronic device with a microlithography exposure apparatus according to claim 13 wherein a mask with a structure in the object plane is illuminated and said mask is imaged onto a light sensitive object situated in the image plane. 